1. Field of the Invention
The present invention relates to self-locking fasteners and, in particular, to bolts, nuts, washers and the like having serrated bearing surfaces that provide resistance against the fastener becoming loosened unintentionally after it has been seated. The self-locking fastener of the present invention can be used to particular advantage for workpieces having relatively soft bearing surfaces whereby marring of the workpiece surface is minimized.
2. Description of the Prior Art
A serious problem in joints secured together by threaded members is the possibility of joint separation due to a nut backing-off from a bolt or a bolt backing-out from a nut or other internally threaded member. Generally, this result can occur when the joint is subjected to vibrations.
Various proposals have been suggested in the past to either eliminate or greatly reduce the unintentional loosening of threaded members. Because of the wide variety of applications in which this undesirable result can occur, many different types of locking devices have been developed. One approach has been to treat the bearing surface of the fastener in such a manner that the resistance to relative rotation between the fastener and a workpiece in which the fastener is installed is greater than the resistance to relative movement between the mating threads. As a result, the resistance to rotation between the mating threaded parts no longer is the critical factor in determining whether the threaded parts will turn relative to each other.
One important requirement of these fasteners is that the "off" torque (torque required to loosen a tightened fastener) be greater than the "on" torque (torque required to seat a fastener properly.)
Because the bearing surface of these fasteners, for the most part, are serrated or are provided with teeth or the like which are arranged to dig into the workpiece to create resistance to relative rotation between the fastener and the workpiece, some damage or marring of the workpiece bearing surface will occur as these fasteners are seated and removed. Such damage to the workpiece causes it to weaken. Hence, a second important requirement of these fasteners is that the effect of marring or damage to the workpiece is minimized.
Generally, most prior art fasteners provided with a locking characteristic in the bearing surface fail to satisfy concurrently these and other requirements. Those fasteners available at the present time having improved "off" torque to "on" torque ratios dig into the workpiece in such a manner or to such an extent as to weaken greatly the workpiece. "Notch" effect (the build up of stress concentrations) is a common result and may cause fatigue failure of the clamped parts. This problem becomes more acute as the thickness of the workpiece is reduced. Those fasteners available at the present time having reduced adverse effect on the workpiece provide insufficient "off" torque.
The drawbacks of this type of prior art fastener are overcome in a fastener such as that disclosed in commonly assigned U.S. Pat. No. 3,605,845. This patent discloses a self-locking fastener which provides a resistance to unintended rotation between the fastener and the workpiece and yet causes a minimum amount of marring of the workpiece surface with which the fastener bearing surface is in contact. This is accomplished by providing a smooth surface annular ring about a plurality of radially disposed serrations which, upon engagement with the workpiece, opposes further penetration of the serrations and controls the extent of penetration of the serrations.
A similar improved fastener is also disclosed in commonly assigned U.S. Pat. No. 3,825,051. This patent discloses a self-locking fastener having a polygon shaped workpiece bearing surface, such as a hex-head, where the serrated segment is formed within an annular segment so that circumferentially discontinuous smooth-faced outer bearing surfaces are formed across adjacent flats of the polygon configuration.
Thus, it is an object of the present invention to provide a new and improved self-locking fastener having a locking mechanism included in the bearing surface of the fastener and which is also provided with a stress regulating configuration within the locking mechanism to control the extent of penetration of the fastener into the workpiece.
It is another object of the present invention to provide a self-locking fastener of this type which provides improved resistance to unintended rotation between the fastener and a workpiece and yet causes a minimum amount of marring of the workpiece surface with which it is in contact.
It is a further object of the present invention to provide a self-locking fastener which is relatively simple in construction and inexpensive to fabricate.